Cryogenic ship containment system having a convection barrier

ABSTRACT

A ship for the transporation of volatile liquids having holds which contain a number of elongated vessels for containing cargo fluids where each vessel has a primary barrier for isolating cargo fluids from the hull and has an insulating wall. The insulated cargo vessels prevent the absorption of heat by cargo fluids at cryogenic temperatures which could cause excessive loss of cargo by evaporation and prevent a thermal equilibrium temperature of cargo fluids at cryogenic temperatures from affecting the hull of the ship, e.g., by preventing embrittlement of the hull. A convection barrier connects at least each pair of adjacent vessels positioned opposite the surfaces of the hold to decrease thermal conductivity between cargo fluids in the vessels and the hull structure of the ship.

United States Patent [191 Bolton [111 3,830,180 Aug. 20, 1974 [75] Inventor: Harold B. Bolton, Marina Del Rey,

Calif.

[73] Assignee: Litton Systems Inc., Beverly Hills,

Calif.

[22] Filed: July 3, 1972 [21] Appl. No.: 268,436

52 US. Cl 114/74 A [51] Int. Cl B63b 25/08 [58] Field of Search 114/74 A, 74 R, 74 T [56] References Cited UNITED STATES PATENTS 2,687,618 8/1954 Bergstrom 114/74 A 2,810,265 10/1957 Beckwith..... 114/74 A 3,270,700 9/1966 Kohn et al. 114/74 A CRYOGENIC SHIP CONTAINMENT SYSTEM HAVING A CONVECTION BARRIER Primary ExaminerTrygve M. Blix Assistant ExaminerStuart M. Goldstein Attorney, Agent, or Firm-Alan C. Rose ABSTRACT A ship for the transporation of volatile liquids having holds which contain a number of elongated vessels for containing cargo fluids where each vessel has a primary barrier for isolating cargo fluids from the hull and has an insulating wall. The insulated cargo vessels prevent the absorption of heat by cargo fluids at cryogenic temperatures which could cause excessive loss of cargo by evaporation and prevent a thermal equilibrium temperature of cargo fluids at cryogenic temperatures from affecting the hull of the ship, e.g., by preventing embrittlement of thev hull. A convection barrier connects at least each pair of adjacent vessels positioned opposite the surfaces of the hold to decrease I thermal conductivity between cargo fluids in the vesselsand the hull structure of the ship.

1 Claim, 12 Drawing Figures "mnzemJmEAm-w PATENTEB CRYOGENIC SHIP CONTAINMENT SYSTEM HAVING A CONVECTION BARRIER CROSS-REFERENCES TO RELATED APPLICATIONS This Application is related to the following patent application filed this same day:

1. System for Containing and Discharging Liquified Gases by Stoddard Waldron;

2. Stress-Relieved Vessel for Storing Liquified Gases by Harold B. Bolton; and

3. Cryogenic Ship Containment System by Robert E. Apple.

FIELD OF THE INVENTION This invention pertains to the art of building ships for the transportation of volatile liquids and, more particularly, ships for transporting liquified natural gases.

DESCRIPTION OF THE PRIOR ART The cargo tankers of the prior art for transporting liquified natural gases may be classified generally as either having free-standing tanks or membrane tanks. The classification of ships having free-standing tanks may be further divided into tankers having prismatic freestanding tanks and those having spherical free-standing tanks.

LNG tankers of the prior art having prismatic or spherical freestanding tanks or membrane tanks are subject to several disadvantages. One such disadvantage is a high initial cost of construction. Although freestanding tanks may be constructed remotely from the construction site of the ship, such tanks have a high cost for the material from which the tanks are built. This material may be an aluminum alloy, nickel steel or an alloy not subject to embrittlement at cryogenic cargo storage temperature.

All the insulation systems of LNG ships of the prior art are subject to the disadvantage that their insulation systems are inefficient in that there is a loss of cargo due to the influx of heat to the cargo and subsequent boil-off. The tanks fill virtually all or at least a large portion of a hold. If inspection reveals damage or a structural failure, the tanks cannot easily be repaired. Removal of such a tank from a ship is virtually precluded because the tanks are an integral part of the hull structure, which results in prohibitive repair costs.

Tanks of the LNG ships of the prior art are all to some extent subject to damage during construction. This is particularly true for tanks of the integral membrane tanks.

The tanks are vulnerable in the event of failures such as fractures or ruptures caused by a collision. Again, this is particularly true of membrane tanks. The membrane tank is a tank of large volume not usually loadbearing and is connected to the hull and supported by an insulation system. Free-standing tanks of the prior art ships are also tanks of large volume, although not directly connected and supported on the hull. In the event of a collision, either type of tank would release a tremendous volume-of combustible volatile fluids. Release of a volatile fluid such as LNG from a fracture involves a great amount of risk due to the cryogenic temperature at which such cargo is carried and the extreme flammability. LNG at cryogenic temperature in contact with ships structural members and hull can cause a brittle fracture leading to uncontrollable release of the LNG.

Tanks of ships of the prior art are subject to the disadvantage that the liquid cargo may have an undesirable free-surface effect on the stability of the ship due to sloshing of liquid cargo.

In addition to the general disadvantage of the LNG ships of the prior art discussed above, each of the above-mentioned types of LNG ships has one or more disadvantages with respect to other types of prior art LNG ships. For example, the prismatic free-standing tank has a higher weight than all other types of tanks.

Free-standing tanks, and particularly spherical tanks, require a larger ship to carry the same volune of cargo as a membrane tanker.

Membrane tankers, on the other hand, are difficult to analyze structurally and reliance must be placed upon fatigue testing of components and on small prototype tanks in order to obtain a measure of reliability.

Membrane tanks having a liquid-type secondary insulation barrier have higher cost for insulation than freestanding tankers which may not require a secondary barrier to contain cargo liquids.

Membrane tanks are subject to the disadvantages that they may be damaged by overpressurization of the space between the membrane and the hull.

One type of LNG ship having free-standing tanks is disclosed in US. Pat. No. 3,270,700. The system disclosed therein utilizes a complicated secondary barrier and insulation. Such complexity will result in corresponding initial cost for insulation and for construction. Also, it appears that because the insulation system is removed a distance from cargo liquids, the temperature gradient between the cargo at cryogenic temperatures and the hull at ambient water temperature would exhibit substantial change in temperature across the hull and its internal structure. Such a substantial temperature difference across the hull and hull girders may require a quality of steel above that of mild steel used in conventional tankers and would correspondingly increase the cost of construction.

The above and other disadvantages of the tankers of the prior art for carrying volatile liquid such as liquidfied natural gases are overcome by the ship of the invention which provides a hull having a number of cargo holds; each cargo hold contains a number of elongated vessels for containing cargo fluids. In the preferred embodiment, each vessel has a primary barrier for isolating cargo fluids from the hull and having an insulating wall for controlling the temperature gradient from the temperature of cargo fluids in the vessel to the temperature or the hull; a convection barrier connects at least each pair of adjacent vessels both opposite the inner surface of a cargo hold for decreasing the thermal conductivity between cargo fluids in each vessel and the hull; each vessel is securely mounted within its hold and is connected to a piping system for selectively filling, venting and discharging cargo fluids.

It is therefore anobject of the invention to provide a ship for carrying volatile fluids having a relatively lower initial cost.

It is another object of the invention to provide a ship for carrying volatile fluids having an insulation system with increased efficiency.

It is a further object of the invention to provide a ship for transporting volatile fluids having a subdivided containment system.

It is also an object of the invention to provide a ship for carrying volatile fluids having a containment system which is less subject to damage occurring during construction, or resulting from a collision or grounding.

It is an additional object of the invention to provide a ship for transporting volatile fluids having a containment system which has a uniquely extensive compartmentation and this is more safe and less susceptible to catastrophic failure.

It is one more object of the invention to provide a ship for the transportation of volatile fluids with reduced free-surface effect.

It is a still further object of the invention to provide a ship for transporting volatile fluids having a containment system of relatively less weight.

It is still one more object of the invention to provide a ship for carrying volatile fluids having a containment system which can be structurally analyzed.

It is yet one more object of the invention to provide a ship for carrying volatile fluids at cryogenic temperatures having a reduced cost for insulation.

It is still an object object of the invention to provide a ship having a containment system for carrying volatile fluids which is not subject to damage due to overpressurization of a space between the containment system i and the hull of the ship.

It is yet another object of the invention to provide a containment system which can be used to convert existing tankers and bulk carriers to ships for transporting volatile liquids.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of a tanker of the invention;

FIG. 2 is a cross section through the tanker of FIG. I at mid-ship;

FIG. 3 is a partial section taken through the hull;

FIG. 4 is a perspective view of a portion of the hull;

FIGS. 5-10 are cross sections of various alternate types of cargo vessels;

FIGS. 11 and 12 are horizontal and vertical cross sections of the same hold showing the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 there is shown a ship for the transporting of liquid natural gas having a hull 12. Hull 12 contains a number of individual holds 14.

FIG. 2 shows a cross section of one hold 14 containing a plurality of vertically disposed subdivision vessels for transporting liquid natural gas and other volatile fluids.

In FIG. 3 there is shown a preferred cargo subdivision structure, comprising an elongated vessel 16 for transporting volatile liquids which has a primary barrier and a secondary insulation barrier surrounding the primary barrier. In the preferred embodiment, vessel 16 is a double-walled vessel of a plastic material having an inner wall 18 and an outer wall 20 and wall or layer 22 of insulation between inner wall 18 and outer wall 20. Each vessel 16 is in the range of 60 to 120 feet high (90 feet being the preferred height) and has an outer diameter of approximately one dozen feet. Inner wall 18 and outer wall 20 are fabricated from a plastic or other suitable material capable of enduring exposure and stress at cryogenic temperatures, such as F RP pipe. Vessel 16 is designed to be self-supporting. Either inner wall 18 or outer wall 30 may be designed to be the primary barrier for containing cargo fluids and for isolating cargo fluids from hull 12 and the inner surfaces of hold 14. In the preferred embodiment, inner wall 18 is the primary barrier.

Wall 22 of insulation comprises a layer of urethane or other suitable insulating materials mechanically or chemically bonded to inner wall 18. The outer wall 20 serves as damage protection when tanks are installed and used. The urethane or other insulation may include filaments of glass reinforcement for distributing stresses throughout the foam and for eliminating voids. Stresses will arise in vessel 16 due to a temperature gradient from inner wall 18 to outer wall 20 due to a difference in the characteristics of the plastic walls and urethane foam or other insulation when exposed to cryogenic cold temperatures. The thickness and selection of insulation materials and layer 22 is amatter of design choice. Increasing the thickness of layer 22 of insulation reduces the cost of cargo losses due to boil-off but, of course, increases the original cost of each vessel 16. For acceptable boil-off rates, layer 22 of urethane foam insulation will have a thickness in the approximate range of 6 to 12 inches.

Each inner wall 18 comprises a cylindrical mid-body section 24, and upper dome 26, and a lower dome, 28, all of homogeneous material. Upper dome 26 is joined to midsection 24 by weld 30. Domes 26 and 28 may be welded to midsection 24 by applying a material of the same strength and elastic modulus to the joint between the cylinder and each dome. The joint may be made in theforrn of a V-notch extending around the circumference of the dome and the midsection with the end of both the dome and the midsection being beveled to form one side of the V-notch. The notch is filled with a homogeneous material of the same material as the midsection and the domes. Although the weld deposit may protrude above the notch and overlap the end of the midsection and the end of the dome, the weld occurs in the material in the notch itself. This welding technique is well-known in the art of welding pipes and tanks. Inner wall 18 is designed to withstand a pressure of approximately 50-60 pounds for reducing losses due to boil-off and for pressurizing the container to blow it empty rather than pumping out the fluid. Of course, vessel 16 may be operated at atmospheric pressure. The inner vessel can be strengthened by application of spirallywound glass filament.

Flanges 35 may be of the same material as outer wall 20 and may be welded to outer wall 20 to provide a means for lifting vessel 16 into hold 14 and for securing it therein. Each flange 35 has an aperture which may be utilized for lifting vessel 16. When vessel 16 is in place in hold 14, its upper end may be secured by a stay 36 adjoining each aperture and each flange 35, as shown in FIG. 4. A pairof flanges 37 may be mounted on each stay 36 on either side of each flange 35 to prevent lateral movement of vessel 16. Flexibility of stays 36 will permit vertical expansion and contraction of vessel 16 due to changes in temperature. Stays 36 may be metal rods and may be affixed to sides of hold 14 in a conventional manner.

Outer wall of each vessel 16 provides a secondary barrier to isolate liquid natural gas or other volatile liquids carried inside inner wall 18 from hull 12.

The bottom of each outer wall 20 is adapted to be mounted in a base which will prevent lateral movement of vessel 16 and which will permit maximum utilization of the volume of hold 14. The bottom of outer wall 20 has a flat portion for resting vessel 16 on the tank top of hull l4. Vessel 16 is seated in support 38 which has a beveled surface for mating with a beveled portion 39 of outer wall 20. Beveled portion 39 of wall 20 angles from the flat bottom of wall 20 to the vertical sides. Base support 38 provides support in both the vertical and horizontal directions. In other words, both vertical and horizontal loads of vessel 16 are transmitted to hull 12 through base support 38. Support 38 may be a ring having vertical sides, a bottom surface contoured to fit any slope in hold 14 and a beveled inner surface to mate with beveled surface 39 of outer wall 20. The outer diameter of support 38 is preferably either equal to or less than the outer diameter of vessel 16 so that vessel 16 may be positioned. Support 38 may be fabricated or cryogenic steel and affixed to the tank top of hold 14 by welding. Alternatively, support 38 may be of the same material as outer wall 20 and affixed to the tank top of hold 14 by metal bolts and nuts (not shown).

Outer wall 20 may be fabricated from a midsection, a lower dome, and an upper dome in the manner discussed above for inner wall 18. The upper end of outer wall 20 tapers into a neck 40 which is coaxial with neck 34 of inner wall 18.

Consider now the single orifice in vessel 16 for filling, venting and discharging. Neck 40 of outer wall 20 terminates in a flange 42. A flange 44 is welded to the end of neck 34 of upper dome 26. A plug 46 having a hold 48 is welded inside the top end of neck 34. Stay tube 50 rises upwardly from the bottom of inner wall 18 and projects through hole 48 in plug 46. Stay tube 50 includes a tubular midsection 52, and upper fitting 54, and a lower fitting 56. Upper and lower fittings 54 and 56 may be cast and are each welded to tubular midsection 52. Tubular midsection 52 is welded to plug 48. Stay tube 50 is supported on base 58 and spacer 60. Base 58 may be cast as a part of lower dome 28. Lower fitting 56 of stay tube 50 is spaced from base 58 by means of spacers 60. Lateral movement of stay tube 50 is prevented by the shear strength of bolts 62 and nuts 64. Each bolt 64 threads in a hole in base 58, a spacer 60, and a hole in lower fitting 56.

Filling and discharging of vessel 16 is accomplished through stay tube 50. Utilization being of stay tube 50 to fill and discharge vessel 16 is discussed in detail below.

Vessel 16 also includes fitting for the venting of boiloff from liquid cargo. Neck 34 of upper dome 26 includes aperture 66 for venting boil-off. A fitting 68 for connecting to a piping system is mounted between flange 44 and flange 42. Fitting 68 is supported by gaskets 70 and 72 and is held in place by bolt 74 and nut 76. Fitting 68 includes a sleeve 78 through which the bolt passes. Boil-off vapor flows out of aperture 66, around sleeve 78 and out of fitting 68. A pair of fittings 68 may be connected to neck 34 on opposite sides to facilitate the venting of boil-off vapor into a piping system.

Operation of vessel 16 as a pressure vessel will impose vertical stress on inner wall 18 as a function of the vapor pressure within the vessel. Stay tube 50, in addition to serving as a means for filling and discharging the vessel, will withstand such vertical stress. Stay tube 50 resists vertical stress because it is firmly connected to both the upper and lower ends of inner wall 18. Vertical stress would otherwise place an undesirable load on welds 30 and 32. Stress may be relieved from welds 30 and 32 by prestressing stay tube 50 by an appropriate amount of force. In other words, stay tube 50 may be connected in tension between plug 46 and base 58.

There is shown in FIG. 4 a plurality of vessels 16 each having a primary and an insulated secondary barrier separated by a layer of insulation, a piping system 80 for filling and discharging the vessel 16, and a piping system 82 for venting vapor from each vessel 16. Cargo fluids are loaded through piping system 80 under pressure. Piping system 80 is also used to discharge cargo fluid. In the preferred embodiment of the invention, cargo fluids are forced from each vessel 16 by the pressure of inert gas applied through the vapor-removal piping system 82. One example of a source of inert gas is a pressure vessel 83 which is isolated from the remainder of the vessels by a pair of valves 84 and 86. Valve 88 is opened to permit boil-off to be exhausted by a suitable means (not shown). An inert gas such as nitrogen may then be loaded into the isolated vessel 83 through pipe 89. To discharge the remaining vessels 16 it is only necessary to close valve 88 of the vaporventing system and to open valves 84 and 81. Valve 84 then permits the nitrogen to exert a pressure on the cargo fluid in the remaining vessels 16. Opening valve 81 permits liquid cargo to flow to a shore-based receiving means (not shown).

Turning now to alternative embodiments of the invention, there are a number of combinations of elongated vessels and insulating'means for controlling the temperature gradient between liquid cargo and the vessels and the temperature of the ships hull that are within the scope of this invention. For example, there are several different types of insulated vessels; there are numerous possible arrangements of cargo vessels utilizing both insulated vessels and uninsulated vessels of the prior art; and numerous possibilities for filling in all or a portion of the void space between the vertical vessels and the inner surfaces of the cargo hold in which the vessels are supported.

In FIGS. 5-9 there are shown five alternate embodiments of insulated vessels. Any one of the vessels shown in FIGS. 59 may be either an atmospheric vessel or a pressure vessel. In FIG. 5 there is shown a vessel comprising a wall 102 and an inner layer 104 of insulation. Vessel 100 may be formed of a pair of end domes and a tubular midsection in the manner described for inner wall 18 of vessel 16 above. Insulation layer 104 is an inner layer in contact with cargo liquid and is impervious to volatile liquids such as liquid natural gases.

In FIG. 6 there is shown another vessel 106 comprising a wall 108 and a layer 110 of insulation. In this embodiment wall 108 is an inner wall in contact with liquid cargo and is the primary barrier. It may be of a material described above for inner wall 18 of vessel 16. Layer 110 of insulation may be urethane foam.

In FIG. 7 there is shown a vessel 112 comprising an inner wall 114 and an outer wall 116. A plurality of spacers 118 of the same material as walls 114 and 116 are utilized to maintain walls 114 and 116 in a spaced relationship.

In FIG. 8 there is shown a vessel 120 having an inner wall 122 amd outer wall 124, similar to the embodiment of FIG. 7, plus an inner layer 126 of insulation such as urethane foam. Walls 122 and 124 are maintained in spaced relationship by a plurality of spacer rings 128.

In FIG. 9 there is shown a vessel 130 having an inner wall 132 and an outer wall 134, similar to the embodiment of FIG. 7, plus a layer 136 of insulation which is outside outer wall 134. Walls 132 and 134 are maintained in spaced relationship by a plurality of spacer rings 138.

Each of the embodiments of FIGS. 6-9 may be constructed in the manner described above for inner wall 18, outer wall 20, and layer 22 of insulation of vessel 16. Either the inner wall or the outer wall may be designed to be the primary barrier, preferably the inner wall.

In FIG. 10 there is shown a cross section of an elongated vessel 140 which has no insulation. Such vessel may be utilized in conjunction with any combination of vessels 16, 100, 106, 112, 120 and/or 130 according to the teachings of the invention in the manner described below.

In FIGS. 11 and 12 there is shown yet another embodiment of the invention wherein the outer group vessels around the periphery of a hold 14 are vessels having insulated walls such as vessel 16, or, alternatively, 100, 106, 112, and/or 130. The outer group of vessels surround the inner group of vessels 142.

Each vessel 142 may be an insulated vessel such as vessel 16, or the other above-mentioned insulated vessels or, alternatively, it may be an uninsulated vessel such as vessel 140. Where vessels 142 are insulated vessels, insulation between the vessels may be omitted, as shown in FIG. 11. Each insulated vessel 16 is connected to adjacent insulated vessel 16 by a convection barrier 144. Convection barrier 144 comprises a rib which may be of the same cryogenic material such as plastic from which vessels 16 are made. Convection barriers 144 should be somewhat flexible to permit the vertical expansion of each vessel 16 as a function of temperature changesuFIG. 12 is a vertical cross section of the hold 14 of FIG. 11 showing convection barriers 144, a top layer of insulation 146 covering the vessels and a bottom layer of insulation 148 insulating the vessels 16 and 142 from the tank top of hold 14. The insulated vessel 16. and convection barriers 144 control the temperature gradient between the temperature cargo fluids in vessels 16 and 140 and the temperatures of hull l2 and internal structures betweenhull l2 and the 8 cargo vessels. Thermally isolating the cargo vessels l6 and permits the use of mild steel for hull 12 and internal hull structures without danger of embrittlement of the steel by exposure to cryogenic temperatures.

Each convection barrier 144 may be cast from a foam material such as urethane foam in a shape which interfaces and fits the contour of each vessel 16. Each convection barrier 144 may be held in place between a pair of adjacent vessels 16 by a suitable adhesive. Convection barriers 144 decrease thermal conductivity between cargo fluids in all but the outer group of cargo vessels directly opposite a surface of hold 14 and the structures of hull 14. Thermal conductivity is reduced because air or other gas circulating around these vessels inside the outer ring of vessels joined by convection barriers cannot reach the surfaces of hold 14 and thus cannot affect the temperature of the hull structures by convection.

Of course, the vessels 142 may also be of the type of uninsulated vessels 140 shown in FIG. 10 and discussed above.

I claim:

1. A ship for transporting volatile fluids at cold temperatures including cryogenic temperatures comprismg:

a. a hull structure having at least one cargo hold;

b. a first plurality of first cargo elongated vessels disposed in said hold and spaced from one another, each said first cargo vessel in said first plurality having at least a pair of walls, one of said walls comprising a primary barrier of cryogenic material and the other comprising an insulating material, said first plurality of first cargo vessels being vertically disposed in at least one said hold in a predetermined pattern around the periphery of said hold;

c. a second plurality of second elongated, cargo vessels disposed in said hold inside the said pattern formed by said first plurality of first cargo vessels, said second vessels being spaced from one another and from said first vessels;

(1. means for supporting each said cargo vessel in said first and said second pluralities of cargo vessels in said hold;

e. heat convection barrier spacing means disposed between each of mutually facing said first elongated vessels whereby a lattice formation is formed around the said periphery decreasing thermal conductivity between cargo fluids in said second vessels and said hull; and

f. means for selectively filling, venting and discharging each of said plurality of cargo vessels. 

1. A ship for transporting volatile fluids at cold temperatures including cryogenic temperatures comprising: a. a hull structure having at least one cargo hold; b. a first plurality of first cargo elongated vessels disposed in said hold and spaced from one another, each said first cargo vessel in said first plurality having at least a pair of walls, one of said walls comprising a primary barrier of cryogenic material and the other comprising an insulating material, said first plurality of first cargo vessels being vertically disposed in at least one said hold in a predetermined pattern around the periphery of said hold; c. a second plurality of second elongated, cargo vessels disposed in said hold inside the said pattern formed by said first plurality of first cargo vessels, said second vessels being spaced from one another and from said first vessels; d. means for supporting each said cargo vessel in said first and said second pluralities of cargo vessels in said hold; e. heat convection barrier spacing means disposed between each of mutually facing said first elongated vessels whereby a lattice formation is formed around the said periphery decreasing thermal conductivity between cargo fluids in said second vessels and said hull; and f. means for selectively filling, venting and discharging each of said plurality of cargo vessels. 